1. Field of the Invention
This invention relates to the processing of semi-conductor wafers. More particularly, this invention relates to the formation of a boron phosphorus silicate glass composite layer on a semiconductor wafer and an improved method of forming same.
2. Description of the Related Art
Boron phosphorus silicate glass (BPSG) has been used as an insulating layer applied over stepped surfaces of integrated circuit structures formed on semiconductor wafers. BPSG is used over stepped surfaces because of its low melting temperature which permits subsequent heating of the layer to flow the glass to thereby planarize the structure.
Various processes have been used to form the BPSG layer over the stepped surface on the semiconductor wafer. A plasma-assisted chemical vapor deposition (CVD) process using silane (SiH.sub.4) together with boron and phosphorus doping materials such as, for example, trimethylphosphate (TMP) and trimethylborate (TMB), has been used in the past.
However, one problem which can occur when forming such a layer over stepped surfaces resulting from, for example, closely spaced apart raised lines or narrow trenches, is that of the formation of voids.
Referring to FIG. 1, which shows a prior art formation of a boron phosphorus silicate layer 30, an integrated circuit structure 10 is shown having trenches 24 and raised lines 20 thereon which may, for example, be metal or polysilicon lines.
When such a prior art BPSG layer 30 is applied over the structure, void formation, indicated at 34, occurs in the cavities between closely spaced apart lines 20 or in narrow trenches 24 when the material of BPSG layer 30 being formed deposits or grows on the sidewalls of the cavities, comprising either the space between adjacent lines 20 or the space in trenches 24, faster than it grows or deposits on the bottom of the cavities so that the side surfaces neck together at or near the top of the cavity before the cavity is completely filled, leaving voids or unfilled spaces 34 in the cavity which, upon subsequent sectioning of the wafer, can be visually seen by a 2000.times. power microscope.
When a plasma-assisted CVD silane process, such as described above, is utilized in the construction of VLSI integrated circuit structures, such as 1 megabit DRAMs, where spacing between metal lines is about 1.2 microns or less, only about 30% to 50% of the resulting step coverage is free of such voids.
With the increasing density of integrated circuit structures, for example, in the construction of 4 and 16 megabit DRAMs, the step coverage of a BPSG layer formed by such a plasma-assisted CVD silane process is unacceptable. Because of this, improved processes have been developed for forming BPSG insulating layers over stepped semiconductor wafers having fine pitch lines or high aspect ratio trenches, e.g., 0.5 to 1 micron spacing between lines or sidewalls of trenches.
One such process is a plasma-assisted CVD process which utilizes tetraethylorthosilicate (TEOS) as the source of silicon together with O.sub.3, and boron and phosphorus dopants to form the BPSG layer. While this process results in better step coverage, the result is still only about 70% void-free coverage of lines having 0.8 to 1 micron spacing such as found in 4 megabit DRAMs.
A CVD process which uses tetraethylorthosilicate as the source of silicon and which does not utilize a plasma, but uses pressures ranging from 60 Torr up to atmospheric pressure (760 Torr), has also been developed for depositing BPSG layers. Use of this process has resulted in 100% void-free step coverage over lines spaced as close as 0.5 microns. Typically, formation of a BPSG layer with a non-plasma-assisted process includes the further steps of densifying the CVD formed BPSG layer at temperatures of about 700.degree. C., followed by a wet etch to clean the wafer, and then a final high temperature flow step to planarize the structure.
The densification step, however, takes time and also generates undesirable particles, while the wet etch step preferentially leaches out boron from the BPSG layer and attacks the sidewalls of the steps or trenches. Furthermore, the BPSG layer formed by this prior art process has high tensile stress, resulting in cracks in the layer. The BPSG layer formed by this process also is hygroscopic, and moisture adsorbed into the layer can result in conversion of the boron dopant in the glass to B.sub.2 O.sub.3 or even recrystallization of the boron to boric acid.
There remains a need for a BPSG layer formed on a stepped surface of a semiconductor wafer, and an improved method for making same, wherein voids will not be formed in the BPSG layer as it deposits in the areas between closely spaced raised lines and/or high aspect ratio trenches, and wherein the surface of the BPSG layer is not hygroscopic or in other ways subject to loss of boron from the layer which could, otherwise, affect the ability to flow the glass to planarize the structure.